Ontogeny recapitulates phylogeny: Fetal sloth teeth point to evolutionary history

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Sloths are strange creatures in many ways. In addition to their slow movements and suspensory way of life, they have a chambered stomach like a cow, algae species that are found on their hair and nowhere else in the world, moths whose life cycles are entirely tied to sloths (“sloth moths”), surprisingly degenerate vision, and strange teeth.

Their teeth are bizarre in a number of ways: they lack enamel, they don’t have baby teeth (i.e., they are born with the only set of teeth they will ever have), and their teeth are notoriously difficult to categorize. By this I mean that scientists aren’t really sure if sloths have canine teeth or premolars. What does seem clear is that they do not have incisors, the front teeth typically present in mammals.

Based on evolutionary theory, scientists think that sloths descended from mammals that had all of the typical mammalian tooth traits, leading to the prediction that they may retain some features suggestive of this earlier dental history. For example, despite lacking enamel, sloths retain defunct remnants of enamel genes in their genomes. The ontogeny, or development, of sloths might give more hints of their toothier past.

Lionel Hautier and his colleagues [1] set out to examine sloth fetuses using CT scans to test this idea. After examining 25 sloth specimens, they found that sloths had very tiny teeth that are not present in adults. At least one set of teeth in the two-toed sloths (Choloepus) are replaced, suggesting they do indeed have baby teeth, but they do not remain once the sloths are born.

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dCf stands for deciduous, or baby, teeth. Note that the two-toed sloth fetuses (A, B) have baby teeth (dCf) right next to the adult teeth (Cf) before the former disappear entirely when they’re born. C is a three-toed sloth for comparison.

Additionally, the researchers discovered that some of the tiny teeth in the three-toed sloths are incisors, which characteristically develop on the premaxilla of the upper jaw (red coloring in the images above and below). However, these appear to be resorbed by the time of birth. Interestingly, not all of the sloths develop with these tiny incisors, suggesting that they may currently be evolving such that they will eventually be completely lost.

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Three-toed sloth fetuses, some of which (C,D) have incisors (dVpmx) that are not retained in the adults, as well as other teeth that disappear before birth (dv).

Questions for Creationists

Why do sloths have teeth that begin to develop and then disappear altogether? Would it not have been simpler for God to create sloths with only one set of teeth and not teeth that no longer function? Is it just a coincidence that sloth fetuses replace their baby teeth with adult teeth and have incisors, just like early fossil mammals and most mammals today? Why do we also see a similar pattern in baleen whales, which lack teeth entirely as adults?

References

1. Hautier, L., Rodrigues, H. G., Billet, G., & Asher, R. J. (2016). The hidden teeth of sloths: evolutionary vestiges and the development of a simplified dentition. Scientific reports6.

Photo credit

Three-toed sloth, deciduous teeth, incisors

 

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